This invention relates to an apparatus for the determination of the fill state of solid or liquid material in a container with a signal generating/transmitting unit which generates high-frequency signals with a predetermined transmission repetition frequency and transmits them in the direction of the surface of material. The high-frequency signals are reflected at the surface of the material and a receiving unit receives the reflected signals. A time-delay circuit transforms the high-frequency signals into low-frequency signals and an evaluating unit determines the fill state in the container using the transit time of the signals.
Processes for the determination of the fill state using the transit time of signals exploit behavior following the laws of physics according to which the transit interval is equal to the product of the transit time and propagation speed. In the case of fill state measurement the transit interval corresponds to twice the distance between the antenna and the surface of the material. The actual effective echo signal and its transit time are determined with the aid of the so-called echo function, the digital envelope curve or the low-frequency signals where the envelope curve or the low-frequency signals reproduce the amplitudes of the echo signals as a function of the distance from the antenna to the surface of the material. The fill state itself can then be determined from the difference between the known distance of the antenna from the base of the container and the distance of the surface of the material from the antenna as determined by the measurement.
In DE 31 07 444 A1 a high-resolution pulse radar process is described. A generator generates first microwave pulses and radiates them via an antenna with a predetermined transmission repetition frequency in the direction of the surface of the material. An additional generator generates reference microwave pulses which are equal to the first microwave pulses but differ slightly from them in their transmission repetition rates. The echo signal and the reference signal are mixed. At the output of the mixer an intermediate frequency signal is present. The intermediate frequency signal has the same curve as the echo signal but is extended with respect to it by a time-delay factor which is equal to a quotient of the transmission repetition factor and the frequency difference between the first microwave pulses and the reference microwave pulses. For a transmission repetition frequency of several megahertz, a frequency difference of a few hertz, and a microwave frequency of several gigahertz, the frequency of the intermediate frequency signal lies below 100 kHz. The advantage of using the intermediate frequency is that relatively slow and therefore cost-effective electronic components can be used for the monitoring of signals and/or signal evaluation.
Determination of the time extension or time delay by means of sequential sampling assumes that the time differential between two sampling points is always the same. Until now two processes were known which are suitable for the satisfying of this requirement: the mixer principle and the ramp principle. The ramp principle quantizes and works only approximately continuously.
In the case of the mixer principle two oscillators generate two oscillations with slightly different frequencies. By the slight "detuning" of the two oscillations a phase shift, which increases linearly with each period, arises which corresponds to a linearly increasing time delay.
A disadvantage of the mixer principle is the relatively high power consumption so that the power supply of a 4-20 mA current loop is only possible in the case of measurement rates of one measured value per second. Furthermore, disproportionately high demands must be made on the hardware and software in order to maintain the same time differential between the sampling points.
The mixer principle provides a comparatively small number of measured values per meter for sequential sampling. The result of this is low sensitivity of measurement for measurements with microwave pulses.
In the case of the ramp principle the same time differential from sampling point to sampling point is generated with the assistance of an RC circuit. The RC circuit is preset by a step voltage or linearly increasing ramp voltage, and therefore receives a certain offset, and is then charged and discharged in the rhythm of the transmission repetition frequency. The voltage offset in the RC circuits increases with increasing ramp voltage whereby reaching the operating point is delayed as a function of the level of the ramp voltage.
In the case of the ramp principle the time extension factor is critically dependent on the time constant of an RC circuit. The dependence of the RC circuit on the temperature has, depending on circuit technology, a great effect on the scaling of the time transformation. In order to eliminate this problem it is necessary to compensate for the temperature-dependent changes via a control circuit. However, even with this control circuit the scaling of the time transformation changes with the temperature in such a way that requirements for highly precise measurements are not fulfilled. A disadvantage of the ramp principle is furthermore the high expenditure which must be made for the generation of the ramp voltage.